The present disclosure relates to systems and methods for purifying a liquid feed stream by removing various solutes from the liquid feed stream to obtain a purified permeate, or output stream. In particular, such methods are contemplated to be useful in desalination of saltwater and concentration of brines. Multiple reverse osmosis stages are arranged in a cross-current osmotically assisted reverse osmosis (OARO) arrangement that permits the use of reduced pressures and overall lower energy consumption compared to traditional reverse osmosis processes. Additionally, the arrangement allows the application of such systems to streams that are concentrated beyond what is typically practical for reverse osmosis.
An osmotic membrane is semi-permeable barrier which allows smaller molecules (usually solvent molecules such as water) to pass through while blocking the passage of relatively larger molecules or ions (e.g. solutes dissolved in the solvent). Osmotic pressure is generally related to the difference in solute concentration between the liquids on either side of the osmotic membrane. Reverse osmosis (“RO”) is the process of applying pressure to the side of the osmotic membrane containing the higher solute concentration in excess of the osmotic pressure, which serves to drive the osmotic process in reverse, i.e. drives solvent through the osmotic membrane to the side with a lower solute concentration. RO is suitable for a wide range of applications, especially when salt and/or dissolved solids need to be removed from a solvent, and is commonly used for desalination.
To achieve increased solvent separation, higher driving pressure must be applied. However, due to operational pressure limitations of many available osmotic membranes, increased driving pressure is not always feasible. Other methods have been used, such as distillation (e.g., multiple stage flash, vacuum, etc.); mechanical vapor recompression; thermal vapor recompression; forward osmosis; and/or single or multi-stage evaporators. These methods are typically all relatively energy intensive compared to reverse osmosis. Other methods have relied on the use of multiple interstage pumps, valves, bypasses from low pressure to high pressure sides of membranes, and plumbing to mix streams of similar concentrations so as to improve process efficiency. However, in practice, such setups are complicated to run, especially during startup and process upsets.
There remains a need for less complex reverse osmosis processes and systems which permit purification of a liquid, but operate at reduced pressures and/or have reduced overall system energy consumption, and can be used with liquids having both high and low solute concentrations.